Display terminal system

ABSTRACT

A system of display terminals each receiving from an image generator edges which commonly represent a selected twodimensional projection of three-dimensional objects. Each edge is defined by its intended position on a two-dimensional screen and by attributes for a portion of the screen adjacent the edge. These attributes may include color, brightness, shading and the like. Each display terminal has an edge memory which stores a set of edges ordered by the y-coordinate of their tops and are partially x-ordered. The stored edges are decoded into scanline segments, each segment being the scanline interval between two adjacent edges intersecting that scanline. Each segment is defined by its intended position on the display screen and by its other attributes such as color brightness. The segments are subsequently decoded into video raster points, each point being defined by its position on the display screen and by attributes such as brightness and color. Since only edges are transmitted from the image generator to the terminals, as opposed to a video raster, a relatively low capacity communication link can be used. Since edges are buffered at each display terminal, rather than video raster data, the edge memories inside the terminals need not be excessively large. The conversion of edges to segments and of segments to video raster is fast and efficient, and allows flickerless display of complex color images.

' United States Patent Swallow 5] Dec. 9, 1975 DISPLAY TERMINAL SYSTEMimage generator edges which commonly represent a 75 I t I R al selectedtwo-dimensional pro ection of threenven or on d J Swallow Upperdimensional objects. Each edge 18 defined by its in- Marlboro, Md.

tended position on a two-dimensional screen and by [73] A lgn R searrporation, New York, attributes for a portion of the screen adjacent theN.Y. Filed: Apr. 18, 1974 Appl. No.: 462,171

Primary ExaminerMarshall M. Curtis Attorney, Agent, or FirmCooper,Dunham, Clark, Griffin & Moran [57] ABSTRACT A system of displayterminals each receiving from an edge. These attributes may includecolor, brightness, shading and the like. Each display terminal has anedge memory which stores a set of edges ordered by the y-coordinate oftheir tops and are partially xordered. The stored edges are decoded intoscanline segments, each segment being the scanline interval between twoadjacent edges intersecting that scanline. Each segment is defined byits intended position on the display screen and by its other attributessuch as color brightness. The segments are subsequently decoded intovideo raster points, each point being defined by its position on thedisplay screen and by attributes such as brightness and color. Sinceonly edges are transmitted from the image generator to the terminals, asopposed to a video raster, a relatively low capacity communication linkcan be used. Since edges are buffered at each display tenninal, ratherthan video raster data, the edge memories inside the terminals need notbe excessively large. The conversion of edges to segments and ofsegments to video raster is fast and efficient, and allows flickerlessdisplay of complex color images.

36 Claims, 21 Drawing Figures comm M4 7/0 Sheet 8 of 15 3,925,776

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Tim 1 BLOCK D/AGRAM OF LEVEL 5 U.S. Patent Dec.9, 1975 Sheet 14 of 153,925,776

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DISPLAY TERMINAL SYSTEM BACKGROUND OF THE INVENTION The invention is inthe field of computer graphics and relates specifically to a system ofdisplay terminals for displaying graphical information received from acentral image generator.

In computer graphics, a representation of a generally three-dimensionalworld is stored in a memory, and selected two-dimensional projections ofselected portions ing as a memory and storing a selected set oftwo-dimensional views of a three-dimensional world, combined with atelevision set. A more complex example is a specially programmed digitalcomputer system which stores a three-dimensional object such as a cubeby identifying the coordinates of its edges, generates selectedtwo-dimensional projections of the three-dimensional object and thecoordinates of the lines making up the projection, and either transmitsthe coordinates of the lines representing the projection to a stroketype display device or converts the line coordinates of the projectionto a set of raster points and transmits that set to a raster displaydevice such as a television receiver.

A survey of computer graphic techniques may be found in Sutherland,I.E., A Characterization of Ten Hidden Surface Algorithms, ACM ComputingSurveys, Volume 6, No. 1, March 1974, pages l55; and in the referenceslisted at page 45 of the article, and particularly in Newman, W.M. etal., Principles of Interactive Computer Graphics, Mc-Graw-Hill, 1973.

A major factor which has prevented widespread use of computer graphicshas been the cost of storing great amounts of information and oftransmitting information to the display devices at a high rate. Forexample, if the purpose of a system is to show on a display surface anyselected view of a three-dimensional object such as a cube, it istheoretically possible to store a nearly infinite number of views on avideo tape and to find and display a selected one on a televisionscreen, but this would be prohibitively expensive. A great reduction inthe amount of stored information results when a three-dimensional objectis stored in computer memory not as it looks in a specifictwo-dimensional view but as it actually is in three-dimensions, e.g., bystoring the three-dimensional coordinates of the apices or the edges ofa cube. This three-dimensional information can be computer-processed togenerate almost any perspective view of the three-dimensional object, tothereby reduce storage costs as compared to a video tape storage. Theremaining question is then how to transmit a representation of the twodimensional view of the object to a display device and how to displayit. The answer to this question must take into account and reconcile anumber of conflicting factors, such as: the desirability of a lowtransmission rate so as to avoid expensive communication links betweenthe central computer and the display device, the desirability of fastoperation of the display device so as to be able to show a complex imagewithout flicker and the desirability of having minimal storage at thedisplay device so as to minimize cost.

In various approaches to reconciling these conflicting factors, someprior art systems use stroke-type display 2 devices (IBM 2250 andTectronics) which form lines by random positioning and stroking of theCRT beam, others use video gating overaTV raster (Anagraph) and stillothers use random point plotting (Plasma). .All

these types of display devices refresh the display either by repeatedimage generation from encoded form such as lines (IBM 2250), by readinga video storage device such as a video disc (Anagraph) or shift registermemory (TICCIT), by reading a storage tube (Tectronics) or by use of aspecial memory display panel (Plasma).

While the stroke type display devices afford relatively low transmissionrate betweenthe images generator and the display device, sincelineidentification data is transmitted and not video raster, these devicescannot display complex images because of limited stroking rates. Whilethe other types of display devices discussed immediately above cantheoretically display complex images, they are severely limited inresolution because of the high cost of locally storing each point of animage and becauseof the required high transmission rate.

There are only two systems known to applicant which use point displaydevices and are'capable of displaying area graphics where arbitraryshapes or surfaces can be simulated: the system developed by the Evansand Sutherland Computer Corporation and the system de veloped by GB. forpilot simulator-like application.

Both are systems in which the terminal displaydevice includes a specialpurpose image generator computer. Both systems generate coloredperspective views from three-dimensional descriptions of objects, butboth are limited to one display device since the output of the imagegenerator is a color video raster to a color monitor. Modification ofthese two systems by the use of a blackboard memory in the displaymonitor, in order to share the image generator for several users, isuneconomical because of the high cost of buffering colored video rasterdata. Additionally, both of these two'systerns are limited in speed,only one picture per second being possible.

A need remained therefore, prior to this invention,

for a graphics display system using a relatively low transmission ratebetween the image generator and the display terminals, capable ofsupporting a number of display terminals by the same image generator,needing minimal storage of data at the display terminal and capable ofproducing flickerless display of complex images, all this at arelatively low price per terminal;

SUMMARY OF THE INVENTION The invention is in the field of computergraphics and relates specifically to a system of display terminalsassociated with an image generator providing information such asdefinitions of selected two-dimensional projections of athree-dimensional world.

The invented system each identified the design goals of relatively lowtransmission rate between the image generator and thedisplay terminals,of relatively small storage capacity at the display terminals and offlickerless display of complex images by (l) transmitting to the displayterminals pictorial information coded as visible two-dimensional edgeseach indentified by its intended position on a two-dimensional displayscreen and by desired attributes of a portion of the display screenassociated with the edge and extending adjacent the edge, such as color,brightness, shading and the like, thus affording relatively lowtransmission ratebetween the image generator and the display terminals,

l/30th t2) buffering the edges at the display terminal. rather thanbuffering video raster information, so as to need relatively low storagecapacity at the terminals, and (3) providing fast but relativelyinexpensive translation of edges to video raster, so that complex imagescan be displayed with no flicker and at low cost.

Each display terminal includes an edge memory for storing a selected setof edges which are y-ordered and x-ordered in a specific manner tofacilitate their conversion to video raster. The edges may be dividedinto blocks, each block including all edges that have portions within aselected span of the screen y-coordinate. Then, the memory may bedivided into a small section, of sufficient size to store a completeblock of edges, and a large section storing the remaining blocks ofedges. In edge decoding, only the small section of the memory has to beaccessed for any given scanline for as long as that scanline belongs tothe span of the block currently residing in the small section in memory.

Each edge defines a continuous line on the screen and selectedattributes for an associated portion of the screen. The line iscontinuous, as opposed to a line defined by series of raster point, andit is a straight line in the disclosed embodiment of the invention,although the invented principles would apply to a curved line as well.The screen portion associated with an edge is the screen area adjacentthe edge and extending to the right of it up to any other edges, but theinvented principles apply to other relationships between an edge lineand an associated screen area, such as a screen portion to the left ofan edge line. The screen portion associated with an edge is a continuousarea, rather than an area defined by a collection of raster points.

Additionally, each edge defines other attributes of the edge line and ofthe associated screen portion, such as brightness, color, brightnessgradient and brightness variation. These attributes provide the inventedsystem with the capability of displaying realistic color and realisticshading so that three-dimensional objects can be simulated on thetwo-dimensional display screen. For example, the edge defines thedesired brightness at the top of the edge line, the color to the rightof the edge line, the gradient of brightness downward along the edgeline, a brightness variation flag which if set means that the color of ascreen portion between two edges will vary linearly, and a flag which ifset means that this is the first edge of a set.

The conversion of edges to video raster takes place in two steps: firstthe edges are converted to scanline segments, each segment being theportion of a scanline disposed between the intersections of thatscanline by edge lines or between an edge line and the end of thescreen, each segment being identified by its position on the screen asby the attributes of the screen portion with which it coincides; andsecond, the scanline segments are converted to a video raster.The'conversion of edges to segments takes place in a pipeline decoderwhose input is the word defining an edge and whose output is a worddefining the position of the segment and the attributes of that segment.Several of the segment defining words output from the edge-to-segmentdecoder are buffered in a ping-pong buffer, and the buffer output isconverted into a video raster which is in color, at a resolution severaltimes that of a conventional TV receiver, and may include smooth shadingfor more realistic depiction of three-dimensional information on atwo-dimensional screen. The output of the segment-to-video decoder isapplied to a commercial color TV receiver modified in accordance withthe invention to operate at a higher resolution and to accept thespecifically formatted output of the segment-tovideo decoder.

A major aspect of the invented display terminals is that they receiveand store edges, rather than video raster, but display a video rasterrather than strokes. By this feature, the invented display terminalcombines the desirable goals of relatively low need for storage capacityat the display terminal and the capability of displaying flickerlesscomplex images. An additional major aspect of the invented displayterminals is that the edges stored at the display terminal are y-orderedand xordered, so that no sorting needs to be done as the edges areconverted to video raster. Still another major aspect of the inventionis the blocking of edges, which reduces the time necessary for cyclingthe edge memory. Still another major aspect of the invention is that itallows for flexibity in defining edges (for example, it is possibleunder the principles of the invention to define curved edge lines and todefine associated screen areas which bear different relationship to thedefined edge line). Still another major aspect of the invention is theprovision for smooth shading of the displayed information, to provide amore realistic appearance of simulated three dimensional objects. Otheraspects of the invention are discussed in the detailed description ofthe invention and are brought out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of theinvented system of display terminals, as connected to a central imagegenerator through a communication controller.

FIG. 2 illustrates an edge definition word.

FIG. 3 illustrates the manner of defining an edge by its position on adisplay screen and by the color and other attributes to the right of theedge until a next edge is encountered, as used in this invention.

FIG. 4 is a block diagram of an edge memory forming a part of eachdisplay terminal.

FIG. 5 is a more detailed block diagram of one bank of the memory shownin FIG. 4.

FIG. 6 illustrates the sorting of a set of edges into blocks of edges inaccordance with the invention.

FIGS. 7, 8, 9, and 10 illustrate the contents of the memory shown inFIG. 4 during different stages of the operation thereof.

FIG. 11 is a generalized block diagram of an edge-tosegment decoder.

FIGS. 12, 13, 14 and 15 are more detailed block diagrams of portions ofthe decoder shown in FIG. 11.

FIG. 16 is a block diagram of a segment buffer receiving the output ofthe edge-to-segment decoder of FIG. 11. 7

FIG. 17 is a partly block and partly circuit diagram of asegment-to-video decoder.

FIG. 18 is a block diagram of a TV receiver modified in accordance withthe invention.

FIG. 19 is a circuit diagram of a portion of an antilog amplifier asused in the embodiment of FIG. 18.

FIGS. 20 and 21 are timing diagrams illustrating the operation of theinvention.

DETAILED DESCRIPTION Overview FIGS. 1, 2, and 3 Referring to FIG. 1, animage generator generates edges each defining the intended location on atwo-dimensional display screen of a line that typically represents aportion of the two-dimensional projection of a three-dimensional object,as well as desired attributes of a portion of the display screenassociated with the edge and extending adjacent the edge.

Referring to FIGS. 2 and 3, each edge may be defined by a 76-bit wordX,Y,H,S,B,C,G,F,E where:

a. X x, an ll bit x-coordinate of the edges top;

b. Y y, an 11 bit y-coordinate of the edges top;

0. H y" y, an l1 bit height, where y is an 1 1 bit I y-coordinate of theedges bottom;

d. S (x x) 2 /(y" y), a 13 bit signed integer (12 bit integer plus 1 bitsign) representing the slope where x" is an 11 bit x-coordinate of theedges bottom and where M is the number of consecutive leading zeros fromthe left in the l 1 bit H up to 10 zeros;

e. B b, where b is the 7 bit brightness (log) at the top of the edge;

f. C 0, color to the right made up of a 4 bit red, a 4 bit blue, and a 4bit green component (each a log function);

g. G (b b) 2 /(yy'), a 9 bit signed integer representing the gradient ofbrightness defined downward along the edge where b" is the 7 bitbrightness (log) at the bottom of the edge;

h. F, a I bit flag which, if zero, implies that the brightness to theright is to vary linearly (in the log domain) toward the value of thenext edge;

i. E, a l bit flag marking the beginning of a set of edges in memory.

Edges may be generated by a system similar to that utilized in thecomputer graphics system of the Evans and Sutherland ComputerCorporation, but since the image generator does not form a part of thisinvention, it may be assumed for simplicity that the image generator 10is a storage device, such as a drum memory or a disc memory, storing aplurality of edges each defined by a 76-bit word of the type shown inFIG. 2, the collection of stored edges representing a desiredtwo-dimensional image, such as a selected two-dimensional projection ofa three-dimensional world. The edges stored in the image generator 10are only those which should be visible on the display screen, and theyare x-ordered and y-ordered according to rules discussed in detailbelow. Additionally the generator 10 stores control information of thetype discussed in detail below.

A plurality of edges of the type shown in FIG. 2 together with eightbits of control information per edge, making up a total of 84 bits peredge, are transmitted via a communication link 11, which may be acoaxial cable of sufficient capacity, to a communication controller 12comprising a receiver 14, a timing generator 16 and a terminal loadcontrol 18. The communication controller 12 serves a number of displayterminals 1, 2 R. The receiver 14 is a serial-to parallel converter forthe edges and control information transmitted from the image generator10, the timing generator 16 provides the necessary timing and controlsignals for transmission to each of the display terminals via a timingbus 20, and the terminal load control 18 decodes the edge data andcontrol data from the coax receiver 14 and ap- 5 plurality of memorybanks and a control for the memory banks, and temporarily stores aselected plurality of edges that are x-ordered and y-ordered in aselected manner; a decoder 26, which comprises an edge-to-segmentdecoder 28, a segment buffer 30 and a segmentto-video decoder 32; and adisplay monitor 34, which may be in color. The purpose of the decoder 26is to convert the edges from the memory 24 into color video raster fordriving the display monitor 34, and it carries out the conversion bydecoding the edges to segments of scanlines, buffering the segments andconverting the buffered segments to a color video raster.

Because the edges are a highly compressed representation of pictorialinformation, the communication link 11 can be of a relatively lowcapacity, and the same image generator 10 can serve up to hundreds ofdisplay terminals, unlike the prior art known to applicant where eachterminal must have its own image genera tor. Because the memory 24 ofeach terminal stores edges rather than video raster, and because of thememory 24 is organized in a novel and efficient manner, it can providesufficient information to the display monitor 34 by storing only a fewthousand edges. Because the memory 24 outputs for each scanline of thedisplay monitor 34 only the edges that intersect it, and because of thenovel and efficient organization of the decoder 26, the decoder canoperate with sufficient speed to ensure flickerless display at themonitor 34 of complex images, and it can operate with a segment buffer28 comprising only a few shift registers.

It is noted that a polygon of arbitrary shape and color can berepresented by edges in accordance with this invention. Because thebrightness may very linearly along the edges bounding the polygon andlinearly in x between edges, a polygon can represent a portion of acurved surface. By means of such polygons, both flat and curved surfacescan be represented in any illumination environment, limited only by thecapacity of the edge memory 24 and by the number of bits used to definean edge. For a planar surface and a point light source at infinity, theedges bounding the left side of a polygon require F to be 1 and G to be0. The result is a polygon of arbitrary but constant brightness andcolor. The F, B and G of the edges bounding the right side of this typeof a polygon do not influence the brightness within the polygon. Planarsurfaces. which are illuminated by neighboring light sources require avarying shading and thus fall into the category of curved shadedsurfaces. The brightness within the polygon is a piecewise linearfunction of that at the comers of the polygon because of the waybrightness is defined along an edge at scanline y by:

b(y) (y Y) G 2"' b (Equation which reduces to My) (y (,v" y) B (Equation2) shaded surface is defined by 7 where .x and x are the x intercepts ofthe two edges and where the form of the equation is that of a linearinterpolation.

The edges on the left of curved shaded polygons have their F set tozero. The edges on the right of these polygons must, of course, have theappropriate Gs and Bs. However, their Fs depend upon the nature of thesurface to their right. If a discontinuity in brightness is required tothe right of a curved shaded surface, the edges on the right of theshaded polygon must be followed by another set of parallel edges whichredefine the brightness and color further to the right.

Aside from the obvious cost advantage afforded by the data compressiondue to edge encoding, there is a second advantage related to imagegeneration. The generation of 2-D perspectives out of 3-D objectsrequires planar approximations to curved surfaces so as to allow lowcost, high speed visible surface calculations. The perspectives of theseplanar surfaces are polygons which are conveniently represented byedges. Also, the brightness of the 3-D objects is generated only for thecomers of the facets representing curved 3-D surfaces where linearinterpolations in the perspective domain are assumed for all otherportions of the surfaces. This interpolation is identical to that usedat the display terminal according to this invention, permitting aperfect match between image generation and display terminal decoding.

Overview of Operation (FIGS. 1-3) Prior to operating the disclosedsystem of display devices, the image generator must be able to provideone or more sets of edges and control data. Although a speciallyprogrammed computer system can be used to provide the necessary data,since the image generator does not form a part of this invention, it canbe assumed for the purposes of this disclosure that the image generator10 is a memory (a drum or disc memory) that is loaded through a keyboard(through an intermediate tape storage, if necessary) with the requireddata. The required data includes one or more sets of edges each definedby a word of the type illustrated in FIG. 2, and each accompanied byseveral bits of control information, e.g. 8 bits. Each set of edges inthe image generator 10 would normally represent a two-dimensionalprojection of a three-dimensional object, but this need not be the case,and the set of edges may represent any suitable pictorial information.The edges of a set are yordered and x-ordered as described in detaillater in this specification. Further, the edges are blocked, again asdescribed in detail later in this specification, and include markeredges defining the blocks of edges and include null edges, as necessary.The several bits of control information accompanying in each edgedefinition word indicate to the terminal load control 18 which of theseveral display terminals is to receive a specific set of edges, whichof the edge defining words already stored in a specific memory 24 of aspecific display terminal should be replaced, updated or deleted, whattiming the timing generator 16 should provide, and the like. Since thesecontrol bits are used to carry out functions which are well known in theprior art, such as loading and updating selected ones of a plurality ofmemories, they are not described in detail in this specification.

In actual operation of the invented system of display terminals, theimage generator 10 delivers over the communication link 11 a set ofedges, e.g. an integer multiple of 1282 edges. As the edges are beingreceived by the communications controller 12, the receiver 14 convertsthe serially received data to parallel data, the timing generator 16,which includes internal clocks, generates the necessary timinginformation for loading the selected memory 24, and the terminal loadcontrol 18 determines which memory 24 receives the edges and directs theedges to that memory 24 in the format suitable for loading it.

The set of edges is loaded in the selected memory 24 in the mannerdescribed in detail later in this specification. Then, theedge-to-segment decoder 28 looks at a subset of the edges stored in thememory 24 (looks at a block of edges) to see which, if any, cross thescanline of the monitor 34 which is currently being decoded. Based ondetecting the edge crossings of'that scanline, the edge-to-segmentdecoder 28 divides the scanline into segments, and defines each segmentby its position on the scanline and by the attributes (color, brightnessand the like) corresponding to the edge crossings bounding the segment.The segment buffer 30 buffers a few of the segments provided by thedecoder 28, and the segment-to-video decoder 32 takes the bufferedsegments and decodes them into a video raster, each point of which isdefined by its position on the screen of the monitor 34 and by itsattributes such as brightness and color. The video raster from thedecoder 32 is applied to the monitor 34 for display at a resolutionseveral times that of the conventional color TV receiver (e.g. 1600points in x and 1200 points in y).

It is noted that while a specific hardware embodiment of the inventionis described in detail in this specification, many variations arepossible within the scope of the invention. For example, edges may bedefined by words differing in format from the words shown in FIG. 2, theedge may define the attributes of a screen area to its left, as well asto its right, or some other associated area of the screen, and differentcomponents may be used for specific functions. Further, parts, or all ofthe invented display terminal system may be simulated on speciallyprogrammed general purpose digital computers.

The invention is described in detail below under the general headingsof: memory 24; decoder 26, including edge-to-segment decoder 28, segmentbuffer 30 and segment-to-video decoder 32; and display monitor 34.

Memory 24 FIGS. 4 and 5 The memory 24 (FIG. 1) is interposed between thememory input bus 22 and the decoder 26, and its purpose is totemporarily store (buffer) a set of edges and to provide to the decoder26 those edges which cross the scanline that is currently being decoded.In order not to have to read out all of the edges in the memory 24during each scanline period (e.g. each 64 microseconds) the memory isorganized into two sections, a small section which is read out once foreach scanline period and a large section which is read out only once foreach vertical trace period (e.g. every l/th of a second) of the display'monitor 34.

Referring to FIG. 4, the memory 24 of each of the display terminalscomprises N banks 1, 2 N. (where N is four, for example) each bankcomprising a large section 36 capable of storing 1,024 edges, and asmall section 38 capable of storing 2 58 edges. The set of the smallsections 38 of the N banks is referred to below as the small section ofthe memory 24 and the set of the N large sections 36 as the largesection of the me m-

1. A system of display terminals, each for displaying a raster patternalong regular scanlines on a two-dimensional screen and each receivingedge definitions, each edge definition comprising signals defining acontinuous, visible line on the screen and a plurality of selectedvisible attributes for a two-dimensional portion of the screen adjacentsaid continuous line, each display terminal comprising: edge memorymeans for storing edge definitions and means for storing in the edgememory means a selected set of edge definitions ordered along at leastone coordinate of the screen; edge-to-segment decoder means forconverting at least a selected subset of the edge definitions stored inthe edge memory means to definitions of one-dimensional segments alongeach of a plurality of defined scanlines across the screen, each segmentdefinition comprising signals defining the position and said attributesof a scanline portion adjacent the intersection of the scanline by anedge line, and segment-to-video decoder means for converting the segmentdefinition signals into videO raster points on the screen, each videoraster point being produced by an electrical signal defined by theattributes of the coinciding segment, to provide on the screen a videoraster representation of at least a selected subset of the set of edges.2. A system as in claim 1 wherein said attribute defined by an edgedefinition include brightness and brightness gradient for the screenportion adjacent the edge line, and said decoder means include means forconverting said brightness and brightness gradient attributes of an edgeinto signals defining corresponding characteristics of correspondingsegments and raster points on the screen.
 3. A system as in claim 2wherein the attributes defined by an edge definition include color forthe screen portion adjacent the edge line, and said decoder meansinclude means for converting the signals defining said color attributesinto signals defining corresponding characteristics of correspondingsegments and raster points on the screen.
 4. A system as in claim 3wherein the attributes defined by an edge definition include a linearbrightness variation flag specifying linear brightness variation betweenthe two ends of a segment disposed between two intersections of itsscanline by an edge line, and the converting means include means forconverting the signals defining said linear brightness variation signalsinto corresponding signals defining corresponding raster points toprovide smooth shading of pictorial information displayed on the screen.5. A system as in claim 4 wherein the set of edge definitions in theedge memory means comprises definitions of a plurality of blocks ofedges, each block including all edges which have at least a portionwithin a selected span of one of the coordinates of the screen andwherein the edge-to-segment decoder means comprise means for convertinginto segment signals the signals defining said blocks of edges.
 6. Asystem as in claim 5 wherein the edge definitions stored in the edgememory means are ordered along each of two coordinates of the screen andthe edge-to-segment decoding means comprise means for converting theedge signals into segment signals in accordance with said order of theedges.
 7. A system as in claim 1 wherein the attributes defined by anedge definition include color for the screen portion adjacent the edgeline, and said decoder means include means for converting the signalsdefining said color attributes into signals defining correspondingcharacteristics of corresponding segments and raster points on thescreen.
 8. A system as in claim 1 wherein the attributes defined by anedge definition include linear brightness variation between the two endsof a segment disposed between two intersections of its scanline by anedge line, and the converting means include means for converting thesignals defining said linear brightness variation signals intocorresponding signals defining corresponding raster points to providesmooth shading of pictorial information displayed on the screen.
 9. Asystem as in claim 1 wherein the set of edge definitions in the edgememory means comprises definitions of a plurality of blocks of edges,each block including all edges which have at least a portion within aselected span of one of the coordinates of the screen and wherein theedge-to-segment decoder means comprise means for converting into segmentsignals the signals defining said blocks of edges.
 10. A system as inclaim 1 wherein the edge definitions in the edge memory means areordered along each of two different coordinates of the screen and theedge-to-segment decoding means comprise means for converting the edgesignals into segment signals in accordance with said order of the edges.11. A system as in claim 1 wherein: the edge memory means include asmall memory section storing a subset of the set of edge definitions anda large memory section storing the remainder of the set of edgedefinitions; the edge-to-segment decoder means comprises means forconverting to sEgments only the edge definitions stored in the smallmemory section; and the storing means comprises means for replacing thesubset of edge definitions stored in the small memory section with asubset of edge definitions obtained from the large memory section afterall edge definitions in the small memory section have been converted tosegments.
 12. A system as in claim 1 wherein the set of edge definitionsin the edge memory means comprises definitions of a plurality of blocksof edges, each block including all edges which have at least a portionwithin a selected span of one of the coordinates of the screen, andwherein the storing means comprises means for storing in the smallmemory section the definitions of a complete block of edges.
 13. Asystem as in claim 1 wherein the edge memory means comprises N banks andthe storing means comprises means for storing the definitions of edges1, N + 1, 2N + 1 . . . in the first bank, for storing the definitions ofedges 2N + 2, 2N + 2 . . . in the second bank, . . . and for storing thedefinitions of edges N, 2N, 2N, . . . of said set of definitions ofedges in the N-th bank.
 14. A system as in claim 1 wherein each edgedefinition comprises signals defining the x, y screen coordinates of oneend of the edge line, the height and slope of the edge line, thebrightness at one end of the edge line and the color for said adjacentportion of the screen, and said decoder means include means forconverting the last recited signals into corresponding characteristicsof corresponding segments and raster points on the screen.
 15. A systemas in claim 14 wherein said adjacent portion of the screen defined by anedge definition is the screen portion on one side of the edge lineextending across the screen up to any other edge lines and said decodermeans include means for converting the last-recited signals intocorresponding characteristics of corresponding segments and rasterpoints on the last-recited portions of the screen.
 16. A system as inclaim 4 wherein the edge definition includes signals defining a gradientof brightness along the edge line and the decoding means includes meansfor converting the last-recited signals into correspondingcharacteristics of corresponding segments and raster points on thescreen.
 17. A system as in claim 16 wherein the edge definition includessignals defining a linear brightness variation for said screen portionadjacent the edge line and the decoder means include means forconverting the last-recited signals into corresponding characteristicsof corresponding segments and raster points on the screen.
 18. A systemas in claim 1 wherein: the edge memory means comprise a plurality ofbanks and the storing means comprise means for storing in each bank asubset of said set of definitions of edges; each bank comprises a smallsection and a large section and the storing means comprises means forstoring in the small section a sub-subset of the subset of definitionsof edges stored in the bank and for storing the remaining definitions ofedges of the subset in the large section; and the edge-to-segmentdecoder means comprises means for converting to segments only thedefinition of edges stored in the small sections of the banks.
 19. Asystem as in claim 18 wherein: the set of edges is divided into blocks,each block including all edges which have a least a portion within aselected span of one of the screen coordinates; and the storing meanscomprises means for storing the definitions of a single block of edgesdistributed among the small sections of the memory banks, said smallsections containing only the definitions of said single block of edges,and for storing the definitions of any other blocks of edges in thelarge memory sections.
 20. A display terminal, for displaying a rasterpattern along regular scanlines on a screen, comprising: edge memorymeans; means for storing in thE memory means a set of edge definitions,each edge definition comprising signals defining a continuous edgeposition on the screen and a set of attributes for an associatedcontinuous, two-dimensional portion of the screen; and means forconverting at least a selected subset of the set of the edge definitionsinto signals defining the points of a video raster displayed on thescreen, each raster point reflecting said attributes for the screenportion with which it coincides.
 21. A display terminal as in claim 20wherein the converting means comprise: edge-to-segment decoding meansfor converting the edge definitions of said subset into signals definingcontinuous segments of scanlines across said screen, each segmentdefined by a position on the screen and by a set of the attributes forthe portion of the screen which which it coincides; and segment-to-videodecoder means for converting each of said segments into signals definingthe points of said video raster.
 22. A display terminal as in claim 21wherein the means for storing the set of definitions of edges in thememory means comprises means for ordering the stored definitions ofedges along a selected first coordinate of the screen.
 23. A displayterminal as in claim 22 wherein the storing means comprises means forstoring the set of definitions of edges in the memory means divided intoa plurality of blocks, each block including the definitions of all edgeswhich have a portion within a selected span of said first screencoordinate.
 24. A display terminal as in claim 23 wherein storing meanscomprises means for storing the definitions of edges in the memory meansordered along a selected second coordinate of the screen.
 25. A displayterminal as in claim 24 wherein the position of an edge and theassociated continuous portion of the screen are adjacent.
 26. A displayterminal as in claim 25 wherein the attributes for the continuousportion of the screen associated with an edge include the color of saidscreen portion.
 27. A display terminal as in claim 26 wherein said setof attributes include brightness and brightness gradient for the edgeposition.
 28. A display terminal as in claim 20 wherein the set of edgesstored in the memory means are ordered along a selected first coordinateof the screen.
 29. A display terminal as in claim 20 wherein the set ofedges stored in the memory means are divided into a plurality of blocks,each block including all edges which have a portion within a selectedspan of a first screen coordinate.
 30. A display terminal as in claim 20wherein the storing means comprises means for ordering the definitionsof edges stored in the memory means along each of two coordinates of thescreen.
 31. A display terminal as in claim 20 wherein the position of anedge and of the associated continuous portion of the screen areadjacent.
 32. A display terminal as in claim 20 wherein the attributesfor the continuous portion of the screen associated with an edge includesignals defining the color of said screen portion and the decoder meansinclude means converting the last-recited signals into correspondingcharacteristics of corresponding segments and raster points on thescreen.
 33. A display terminal as in claim 20 wherein said set ofattributes include signals defining the brightness and brightnessgradient for the edge position and the decoder means include meansconverting the last-recited signals into corresponding characteristicsof corresponding segments and raster points on the screen.
 34. A displayterminal system receiving definitions of edges from a single imagegenerator and comprising a plurality of display terminals, eachcomprising: a display screen; edge memory means for storing definitionsof edges; means for storing in the memory means a set of definitions ofedges, each edge definition comprising signals defining a continuousedge line position on the screen and a set of attributes for anassociated continuous portion of the screen; means for converting atleast a selected subset of the set of definitions of edges into a videoraster, each raster point defined by signals reflecting said attributesfor the screen portion with which it coincides; and means for displayingsaid video raster on the display screen.
 35. A method of operating adisplay terminal for displaying a raster pattern along regular scanlineson a screen comprising the steps of: storing in the display terminal aselected set of definitions of edges ordered along at least onecoordinate of the screen, each stored edge definition comprising signalsdefining a visible continuous line on the screen and selected visibleattributes of a two-dimensional portion of the screen adjacent saidcontinuous line; combining the signals comprising said selected set ofdefinitions of edges to generate definitions of segments along each of aplurality of the scanlines on the screen, each segment definitioncomprising signals positioning a portion of a scanline adjacent an edgeand causing selected brightness and other characteristics of saidscanline portion; and combining the signals defining said segments toderive a sequence of video raster points on the screen to displaythereby on the screen on video raster pattern representing at least aselected subset of the edges and of the adjacent two-dimensionalportions of the screen.
 36. A method of operating a display terminal fordisplaying a raster pattern along regular scanlines on a screencomprising the steps of: storing in a memory means a set of definitionsof edges, each edge definition comprising signals defining a continuousvisible edge position on the screen and a set of attributes for anassociated continuous, two-dimensional portion of the screen; andcombining said edges to generate a sequence of video raster signals forcausing selected video raster points on the screen to form a visiblepattern corresponding to said visible edges and adjacent screenportions.